1. Field of Invention
The present invention relates to well logging techniques. More particularly, the invention concerns an improved telemetry system that uses a periodic pseudorandom training sequence to initialize an adaptive finite impulse response ("FIR") filter-equalizer for optimal communication with downhole measuring equipment, without requiring any changes to the normal logging configuration or any special operator intervention.
2. Description of Related Art
Due to the increasing costs associated with drilling oil wells, well logging has become an important technique to optimize the productivity of oil wells. Generally, in well logging a sensitive measuring instrument is lowered down a borehole, and measurements are made at different depths of the well. In "open hole" well logging, for example, a sonde is lowered down an uncased borehole. The sonde is supported by a cable, which may comprise a monocable or a multiconductor cable wrapped in a steel armor. Multiconductor cables include several individual conductors, which may relay data or electrical power between the surface and the sonde. A typical individual conductor in well logging applications will have a size of about 20 gauge, and may include multiple strands of filaments made from a metallic substance such as copper. One or more conductors usually carry electrical power from the surface to the sonde. In some cases, these conductors carry direct current, and in other cases they carry 60 Hz alternating current. Other conductors of the multiconductor cable carry data from the surface to the sonde, or from the sonde up to the surface. Whether a logging system uses a monocable or a multiconductor cable, a downhole modulator-demodulator ("modem") is used to relay telemetry signals between the sonde and the cable. Likewise, a surface modem is typically used as a telemetry interface between the cable and electrical equipment at the surface.
With open hole logging, a vibrational, electrical, or nuclear source generates disturbances in strata surrounding the borehole, and these disturbances are measured by the sonde. In "production" well logging, an instrument such as a gradiomanometer, densitometer, or capacitance probe is lowered down a cased oil well to measure characteristics of the fluids in the well to determine which depths of the well are producing oil and which are not.
In both open hole and production well logging, the sonde collects information concerning its measurements, and transmits this information to electronic recording and analysis equipment at the surface. The transmission of signals from the sonde to the surface concerns the field of "telemetry." In many cases, proper operation of the telemetry system is one of the most important aspects of a logging system. As a result, geophysicists are constantly striving to improve their telemetry systems. In particular, geophysicists want to receive data from their downhole sondes in a fast and accurate manner. Therefore, it is especially desirable to achieve telemetry systems with fast data transmission rates, as well as high levels of data recognition.
However, improving the data transmission rate in open hole logging systems is limited by the bandwidth of the cable. For example, data on the cable may be attenuated due to the length of the cable. Due to the electrical characteristics of the cable, the signal is distorted by "inter-symbol interference", which refers to a residual signal that appears on a conductor after a data pulse (called a "symbol") has been received (FIG. 1). This type of interference is called "inter-symbol" interference because the residual effect of one symbol often distorts the next, adjacent symbol. In the example of FIG. 1, a signal 100 is transmitted onto a cable (not shown), and a distorted signal 102 is received at the opposite end of the cable. If inter-symbol interference results in a residual signal equal to 65% of the previous symbol, and a 15% residual signal two periods later, the distorted signal 102 will have residual amplitude of 0.65 in an interval 106. In an interval 108, the distorted signal 102 will have an amplitude of 1.15 (i.e., 1.0 due to the symbol received in the interval 108, and 0.15 due to the residual signal from the data pulse received in the interval 104). Moreover, the distorted signal will have an amplitude of 0.65 in an interval 110 (i.e., 0.65 due to the residual effect of the symbol received in the interval 108, with no remaining effect from the symbol received in the interval 104). In an interval 112, the distorted signal will have an amplitude of 0.15, due solely to the residual effect of the symbol received in the interval 108.
In addition to inter-symbol interference, data signals on an individual conductor may be further distorted by noise from data or electrical power carried on other conductors. Moreover, signal distortion may be even more insidious when small diameter conductors are used, or when high temperatures are encountered. Furthermore, the attenuation of data worsens with smaller conductor sizes and increased data transmission rates (FIG. 2).
One technique to overcome inter-symbol interference involves slowing the data transmission rate. This effectively spreads the symbols apart to reduce the "washover" from inter-symbol interference. However, this approach might not be desirable if a fast data transmission rate is needed. Other systems have been developed to help mitigate these problems, as well. One technique, generally called "equalization", utilizes an analog "equalizer" to reverse the effects of frequency-dependent attenuation in telemetry systems. An "equalizer" generally refers to a filter or amplifier that provides selected levels of gain for signals of different frequencies. Many analog equalizers are adjustable (FIG. 3A) to provide various equalization settings for cables of certain expected configurations, e.g. length, diameter, conductivity, etc. By using analog equalizers, the overall amplitude gain of a telemetry system can be made fairly constant over a desired band of frequencies (FIG. 3B).
Although known analog equalizers are beneficial in a number of ways, they are limited in certain other aspects. For example, known analog equalizers are not as adaptable as some people might like, since each setting of an analog equalizer is only designed to operate in one particular logging configuration, i.e., with cable of a specified length conductivity, noise, and other electrical characteristics.
In contrast to analog equalizers, a digital adaptive finite impulse response ("FIR") filter can readily adapt to a wide range of cable types and lengths. However, such filters are not effective until they are properly configured by initializing them, prior to operation, to a reasonably close approximation of their operating configurations. This pre-operation initialization is called "training." In one known training technique (FIG. 4), a logging cable 400 is removed from the borehole, and a surface modem 402 is coupled to the cable 400. A transmitting port 404 of the surface modem 402 is coupled to one end of the cable 400, and a receiving port 406 is coupled to the other end of the cable 400. Then, the transmitting port 404 sends a specified signal to the receiving port 406 via the cable 400. Since the contents of the specified signal and the precise time of sending the signal are known, the relationship between the signals sent and the signals actually received can be analyzed to configure the adaptive FIR filter to accurately interpret the received signals. This technique is addressed in U.S. Pat. No. 5,010,333 ('333), issued on Apr. 23, 1991, to Gardner et al. The '333 patent is hereby incorporated herein by reference in its entirety.
More specifically, with the technique of the '333 patent, the surface modem 402 transmits a long period, pseudorandom signal over the cable 400. This signal is received by the surface modem 402 and compared to the transmitted signal to characterize the effect of the cable 400, and configure the filter appropriately. In particular, the comparison of transmitted and received signals yields error signals, which are processed to determine coefficient signals of the filter. The filter processes received signals and adjusts its coefficient signals until the error signals are minimized. After the error signals are minimized, a delay of about 25 seconds is performed to ensure that the coefficient signals have stabilized. Then the coefficient signals are stored in memory, the cable 400 is disconnected from the receiving port 406, and the cable 400 is connected to a sonde and lowered downhole. Then, the stored coefficient signals are used to initialize the modem 402 in anticipation of receiving data from the sonde.
In some cases, such as the system of the '333 patent, it may be necessary to re-train a filter after the logging cable is placed downhole. This may occur, for example, due to equipment malfunction or replacement. In these cases, the logging cable must be removed from the borehole, which is usually a laborious, expensive process. After the modem is re-trained, the coefficient signals are recorded in memory and the logging cable and sonde are lowered downhole. Then, the stored coefficient signals are used to initialize the surface modem prior to receiving actual data signals from downhole. The set of trained coefficient signals is unique for each different logging cable.
Although many people have found this approach to be sufficient for their purposes, it may be somewhat limited when considered for other applications. For instance, the filter must be re-trained under various circumstances, such as when (1) the logging cable is replaced, (2) the surface modem is replaced, or (3) the original coefficient signals are corrupted, for example, by operator error. Moreover, training performed at the surface may not be as accurate as desired, since the electrical characteristics of the logging cable typically change when the cable is extended downhole, due to high downhole temperatures that are often unpredictable in magnitude and may even vary with depth. In some situations, then, it would be desirable to train the surface modem while the cable is extended downhole, i.e. while the cable is in situ.